arXiv subjects

N. H. Andersen

Publications and source records attributed to N. H. Andersen.

At least 19 recordsLinked to original sources

arXiv2017

Staging superstructures in high-$T_c$ Sr/O co-doped La$_{2-x}$Sr$_x$CuO$_{4+y}$

We present high energy X-ray diffraction studies on the structural phases of an optimal high-$T_c$ superconductor La$_{2-x}$Sr$_x$CuO$_{4+y}$ tailored by co-hole-doping. This is specifically done by varying the content of two very different chemical species, Sr and O, respectively, in order to study the influence of each. A superstructure known as staging is observed in all samples, with the staging number $n$ increasing for higher Sr dopings $x$. We find that the staging phases emerge abruptly with temperature, and can be described as a second order phase transition with transition temperatures slightly depending on the Sr doping. The Sr appears to correlate the interstitial oxygen in a way that stabilises the reproducibility of the staging phase both in terms of staging period and volume fraction in a specific sample. The structural details as investigated in this letter appear to have no direct bearing on the electronic phase separation previously observed in the same samples. This provides new evidence that the electronic phase separation is determined by the overall hole concentration rather than specific Sr/O content and concommittant structural details.

cond-mat.supr-con↗

arXiv2015

Extreme mobility enhancement of two-dimensional electron gases at oxide interfaces via charge transfer induced modulation doping

The discovery of two-dimensional electron gases (2DEGs) at the interface between two insulating complex oxides, such as LaAlO3 (LAO) or gamma-Al2O3 (GAO) epitaxially grown on SrTiO3 (STO) 1,2, provides an opportunity for developing all-oxide electronic devices3,4. These 2DEGs at complex oxide interfaces involve many-body interactions and give rise to a rich set of phenomena5, for example, superconductivity6, magnetism7,8, tunable metal-insulator transitions9, and phase separation10. However, large enhancement of the interfacial electron mobility remains a major and long-standing challenge for fundamental as well as applied research of complex oxides11-15. Here, we inserted a single unit cell insulating layer of polar La1-xSrxMnO3 (x=0, 1/8, and 1/3) at the interface between disordered LaAlO3 and crystalline SrTiO3 created at room temperature. We find that the electron mobility of the interfacial 2DEG is enhanced by more than two orders of magnitude. Our in-situ and resonant x-ray spectroscopic in addition to transmission electron microscopy results indicate that the manganite layer undergoes unambiguous electronic reconstruction and leads to modulation doping of such atomically engineered complex oxide heterointerfaces. At low temperatures, the modulation-doped 2DEG exhibits clear Shubnikov-de Haas oscillations and the initial manifestation of the quantum Hall effect, demonstrating an unprecedented high-mobility and low electron density oxide 2DEG system. These findings open new avenues for oxide electronics.

cond-mat.str-el↗

arXiv2013

Direct proof of unique magnetic and superconducting phases in superoxygenated high-Tc cuprates

We present a combined magnetic neutron scattering and muon spin rotation study of the nature of the magnetic and superconducting phases in electronically phase separated La(2-x)Sr(x)CuO(4+y), x = 0.04, 065, 0.09. For all samples, we find long-range modulated magnetic order below T_N ~ T_c = 39 K. In sharp contrast wit oxygen-stoichiometric La(2-x)Sr(x)CuO(4), we find that the magnetic propagation vector as well as the ordered magnetic moment is independent of Sr content and consistent with that of the 'striped' cuprates. Our study provides direct proof that superoxygenation in La(2-x)Sr(x)CuO(4+y) allows the spin stripe ordered phase to emerge and phase separate from superconducting regions with the hallmarks of optimally doped oxygen-stoichiometric La(2-x)Sr(x)CuO(4).

cond-mat.str-el↗

arXiv2013

A high-mobility two-dimensional electron gas at the heteroepitaxial spinel/perovskite complex oxide interface of γ-Al2O3/SrTiO3

The discovery of two-dimensional electron gases (2DEGs) at the heterointerface between two insulating perovskite-type oxides, such as LaAlO3 and SrTiO3, provides opportunities for a new generation of all-oxide electronic and photonic devices. However, significant improvement of the interfacial electron mobility beyond the current value of approximately 1,000 cm2V-1s-1 (at low temperatures), remains a key challenge for fundamental as well as applied research of complex oxides. Here, we present a new type of 2DEG created at the heterointerface between SrTiO3 and a spinel γ-Al2O3 epitaxial film with excellent quality and compatible oxygen ions sublattices. This spinel/perovskite oxide heterointerface exhibits electron mobilities more than one order of magnitude higher than those of perovskite/perovskite oxide interfaces, and demonstrates unambiguous two-dimensional conduction character as revealed by the observation of quantum magnetoresistance oscillations. Furthermore, we find that the spinel/perovskite 2DEG results from interface-stabilized oxygen vacancies and is confined within a layer of 0.9 nm in proximity to the heterointerface. Our findings pave the way for studies of mesoscopic physics with complex oxides and design of high-mobility all-oxide electronic devices.

cond-mat.mtrl-sci↗

arXiv2013

Crystal Structure and Magnetic Properties of the Ba3TeCo3P2O14, Pb3TeCo3P2O14, and Pb3TeCo3V2O14 Langasites

We report the structural and magnetic characterizations of Ba3TeCo3P2O14, Pb3TeCo3P2O14, and Pb3TeCo3V2O14, compounds that are based on the mineral Dugganite, which is isostructural to Langasites. The magnetic part of the structure consists of layers of Co2+ triangles. Nuclear and magnetic structures were determined through a co-refinement of synchrotron and neutron powder diffraction data. In contrast to the undistorted P321 Langasite structure of Ba3TeCo3P2O14, a complex structural distortion yielding a large supercell is found for both Pb3TeCo3P2O14 and Pb3TeCo3V2O14. Comparison of the three compounds studied along with the zinc analog Pb3TeZn3P2O14, also characterized here, suggests that the distortion is driven by Pb2+ lone pairs; as such, the Pb compounds crystallize in a pyroelectric space group, P2. Magnetic susceptibility, magnetization, and heat capacity measurements were performed to characterize the magnetic behavior. All three compounds become antiferromagnetic with Néel temperatures TN ~ 21 K (Ba3TeCo3P2O14), ~ 13 K (Pb3TeCo3P2O14), and ~ 8 K (Pb3TeCo3V2O14), and they exhibit magnetic transitions at high applied magnetic fields, suggesting intrinsically complex magnetic behavior for tetrahedrally coordinated d7 Co2+ in this structure type.

cond-mat.mtrl-sci↗

arXiv2012

Quantum spin liquid in frustrated one dimensional LiCuSbO4

A quantum magnet, LiCuSbO4, with chains of edge-sharing S = 1/2 CuO6 octahedra is reported. While the Curie-Weiss constant is ferromagnetic, θ= 30 K, no phase transition or spin freezing occurs down to 100 mK. Specific heat indicates a distinct high field phase near the 12 T saturation field. Neutron scattering shows incommensurate spin correlations with q = 0.47\pm0.01π/a and places an upper limit of 70 \mueV on a potential spin gap. Exact diagonalization of easy plane S = 1/2 chains with competing ferro- and antiferromagnetic interactions (J1 = - 75 K, J2 = 34 K) accounts for the T > 2 K data.

cond-mat.str-el↗

arXiv2009

Magnetic ordering in electronically phase separated Sr/O co-doped La(2-x)Sr(x)CuO(4+y)

We present results of magnetic neutron diffraction experiments on the co-doped super-oxygenated La(2-x)Sr(x)CuO(4+y) (LSCO+O) system with x=0.09. The spin-density wave has been studied and we find long-range incommensurate antiferromagnetic order below T_N coinciding with the superconducting ordering temperature T_c=40 K. The incommensurability value is consistent with a hole-doping of n_h~1/8, but in contrast to non-superoxygenated La(2-x)Sr(x)CuO(4) with hole-doping close to n_h ~ 1/8 the magnetic order parameter is not field-dependent. We attribute this to the magnetic order being fully developed in LSCO+O as in the other striped lanthanum-cuprate systems.

cond-mat.supr-con↗

arXiv2009

Anomalous spin-waves and the commensurate-incommensurate magnetic phase transition in LiNiPO4

Detailed spin-wave spectra of magneto-electric LiNiPO4 have been measured by neutron scattering at low temperatures in the commensurate (C) antiferromagnetic (AF) phase with ordering temperature 20.8 K. An anomalous low-energy mode is observed at the modulation vector of the incommensurate (IC) AF phase appearing above the 20.8 K. A linear spin-wave model based on Heisenberg exchange couplings and single ion anisotropies accounts for all the observed spin-wave dispersions and intensities. Along the b axis an unusually strong next-nearest-neighbor AF coupling competes with the dominant nearest-neighbor AF exchange interaction and causes the IC structure.

cond-mat.mtrl-sci↗

arXiv2009

Field-induced magnetic phases and electric polarization in LiNiPO4

Neutron diffraction is used to probe the (H,T) phase diagram of magneto-electric (ME) LiNiPO4 for magnetic fields along the c-axis. At zero field the Ni spins order in two antiferromagnetic phases. One has commensurate (C) structures and general ordering vectors (0,0,0), the other one is incommensurate (IC) with ordering vector (0,q,0). At low temperatures the C order collapses above 12 Tesla and adopts an IC structure with modulation vector parallel to (0,q,0). We show that C order is required for the ME effect and establish how electric polarization results from a field-induced reduction of the total magneto-elastic energy.

cond-mat.mtrl-sci↗

arXiv2005

Phonon-induced quadrupolar ordering of the magnetic superconductor TmNi$_2$B$_2$C

We present synchrotron x-ray diffraction studies revealing that the lattice of thulium borocarbide is distorted below T_Q = 13.5 K at zero field. T_Q increases and the amplitude of the displacements is drastically enhanced, by a factor of 10 at 60 kOe, when a magnetic field is applied along [100]. The distortion occurs at the same wave vector as the antiferromagnetic ordering induced by the a-axis field. A model is presented that accounts for the properties of the quadrupolar phase and explains the peculiar behavior of the antiferromagnetic ordering previously observed in this compound.

cond-mat.supr-con↗

arXiv2005

High-energy Cu spin excitations in PrBa2Cu3O6+x

This paper describes high-energy neutron inelastic scattering measurements of propagating magnetic excitations in PrBa2Cu3O6+x (x approx 0.2 and 0.93). The measurements probe the acoustic and optic modes of the antiferromagnetically-ordered copper--oxygen bilayers in the energy range 50-150 meV. The observed magnon dispersion can be described satisfactorily in this energy range by a spin wave model including intra- and inter-layer nearest-neighbour exchange constants J_parallel and J_perp. We find J_parallel = 127 +/- 10 meV and J_perp = 5.5 +/- 0.9 meV. The value of J_parallel is virtually the same as that found in YBa2Cu3O6.2, but J_perp is a factor of two smaller. To within experimental error the values of J_parallel and J_perp for PrBa2Cu3O6+x do not vary with oxygen doping.

cond-mat.str-el↗

arXiv2004

Phonon Dispersion Relations in PrBa2Cu3O6+x (x ~ 0.2)

We report measurements of the phonon dispersion relations in non-superconducting, oxygen-deficient PrBa2Cu3O6+x (x ~ 0.2) by inelastic neutron scattering. The data are compared with a model of the lattice dynamics based on a common interaction potential. Good agreement is achieved for all but two phonon branches, which are significantly softer than predicted. These modes are found to arise predominantly from motion of the oxygen ions in the CuO2 planes. Analogous modes in YBa2Cu3O6 are well described by the common interaction potential model.

cond-mat.supr-con↗

arXiv2003

Field-induced hexagonal to square transition of the vortex lattice in overdoped La{1.8}Sr{0.2}CuO{4}

We report on a small angle neutron scattering study of the vortex lattice in overdoped La{2-x}Sr{x}CuO{4} (x=0.2) up to high magnetic fields (9.5 Tesla) applied perpendicular to the CuO2 planes. At low magnetic fields we observe a crossover from hexagonal to square coordination of the vortex lattice. This field-induced transition confirms the results obtained in slightly overdoped La{2-x}Sr{x}CuO{4} (x=0.17).

cond-mat↗

arXiv2001

Coupled Magnetic Excitations in Single Crystal PrBa2Cu3O6.2

The dispersion of the low-energy magnetic excitations of the Pr sublattice in PrBa2Cu3O6.2 is determined by inelastic neutron scattering measurements on a single crystal. The dispersion, which shows the effect of interactions with the Cu spin-waves, is well described by a model of the coupled Cu-Pr magnetic system. This enables values for the principal exchange constants to be determined, which suggest that both Pr-Pr and Cu-Pr interactions are important in producing the anomalously high ordering temperature of the Pr sublattice. Measurements of the Cu optic spin wave mode show that the inter-layer Cu-Cu exchange is significantly lower than in YBa2Cu3O6.2.

cond-mat.supr-con↗

arXiv2001

Temperature Dependence of the Flux Line Lattice Transition into Square Symmetry in Superconducting LuNi$_2$B$_2$C

We have investigated the temperature dependence of the H || c flux line lattice structural phase transition from square to hexagonal symmetry, in the tetragonal superconductor LuNi_2B_2C (T_c = 16.6 K). At temperatures below 10 K the transition onset field, H_2(T), is only weakly temperature dependent. Above 10 K, H_2(T) rises sharply, bending away from the upper critical field. This contradicts theoretical predictions of H_2(T) merging with the upper critical field, and suggests that just below the H_c2(T)-curve the flux line lattice might be hexagonal.

cond-mat.supr-con↗

arXiv2001

Antiferromagnetic ordering in superconducting ${\bf YBa_2Cu_3O_{6.5}}$

Commensurate antiferromagnetic ordering has been observed in the superconducting high-$Tc$ cuprate ${\rm YBa_{2}Cu_{3}O_{6.5}}$ (${\rm T_{c}}$=55 K) by polarized and unpolarized elastic neutron scattering. The magnetic peak intensity exhibits a marked enhancement at $T_{c}$. Zero-field $μ$SR experiments demonstrate that the staggered magnetization is not truly static but fluctuates on a nanosecond time scale. These results point towards an unusual spin density wave state coexisting with superconductivity.

cond-mat.supr-con↗

arXiv1999

Interdependence of magnetism and superconductivity in the borocarbide TmNi2B2C

We have discovered a new antiferromagnetic phase in TmNi2B2C by neutron diffraction. The ordering vector is Q_A = (0.48,0,0) and the phase appears above a critical in-plane magnetic field of 0.9 T. The field was applied in order to test the assumption that the zero-field magnetic structure at Q_F = (0.094,0.094,0) would change into a c-axis ferromagnet if superconductivity were destroyed. We present theoretical calculations which show that two effects are important: A suppression of the ferromagnetic component of the RKKY exchange interaction in the superconducting phase, and a reduction of the superconducting condensation energy due to the periodic modulation of the moments at the wave vector Q_A.

cond-mat↗

arXiv1999

Oxygen Ordering Superstructures and Structural Phase Diagram of YBa_2Cu_3O_{6+x} Studied by Hard X-ray Diffraction

High energy x-ray diffraction is used to investigate the bulk oxygen ordering properties of YBa_2Cu_3O_{6+x}. Superstructures of Cu-O chains aligned along the b axis and ordered with periodicity ma, along the a axis have been observed. For $x < 0.62$ the only observed superstructure is ortho-II with m=2. At room temperature we find ortho-III (m=3) for $0.72\le x\le 0.82$, ortho-V (m=5) in a mixed state with ortho-II at x=0.62, and ortho-VIII (m=8) at x=0.67. Ortho-II is a 3D ordered structural phase, the remaining ones are essentially 2D. None of the superstructures develops long range ordering. Studies of the ortho-II ordering properties in samples prepared with x=0.5 but by different methods show that finite size domains with internal thermodynamic equilibrium are formed. The crystal perfection as well as the thermal annealing history determine the domain size. The temperature dependence of the observed superstructure ordering is investigated explicitly and a structural phase diagram is presented.

cond-mat.supr-con↗